It is known to produce a polarizing optical element by combining a polarizing film based on polyvinyl alcohol or PVA with an optical base element. The optical base element is most often an optical lens.
Indeed, polarizing films based on polyvinyl alcohol have been commercially available for a long time and their polarization efficiency is satisfactory. Such polarizing films are conventionally obtained by incorporating dichroic dye molecules and/or dichroic iodine crystals into a polyvinyl-alcohol-based film and then by drawing the film uniaxially so as to orient the dichroic dye molecules and/or the dichroic iodine crystals along the draw direction. The term “dichroic dye” is understood to mean a species that may be of a molecular or crystalline nature and has a preferred absorption of visible electromagnetic radiation for a particular spatial orientation. The polarizing films thus obtained are inexpensive and possess an optical quality compatible with many applications, especially ophthalmic applications. Substrates other than PVA substrates may also be used with this technique of orienting dichroic dyes by uniaxial drawing. For example, polyethylene terephthalate or PET may in particular be mentioned.
A first difficulty in producing an optical element that incorporates a polarizing film lies in how to obtain sufficient and durable cohesion between the optical base element and the polarizing film.
A second difficulty is how to control the position of the polarizing film relative to the faces of the optical element so as not to disturb the path of the light beam through the optical element. This is important for obtaining high uniform optical quality over the entire area for the final polarizing product, insofar as the index of the polarizing film is generally different from the index of the material constituting the optical element. Practically, it is desirable to have excellent parallelism between the face of the optical element that is closest to the polarizing film and the surface of the polarizing film.
To this end, two methods are used for manufacturing an optical element with a polarizing film, depending on the nature of the material of the optical base element.
The first method is used when the optical base element is made of a thermosetting material. The polarizing film, generally preformed to the desired curvature, is placed inside the mold for the optical base element, at a certain distance from two opposed surfaces of the mold. The monomer liquid for the thermosetting material is then poured into the mold on either side of the polarizing film. It is then polymerized by heating the mold containing the liquid and the polarizing film. A polarizing element is obtained, that thus durably incorporates a polarizing film in its thickness. Usually, the polymerization is carried out by heating the mold to a maximum temperature of between 90° C. and 130° C. (degrees Celsius) for a time of about 20 hours. Such polymerization parameters are especially used when the material to be polymerized is diethylene glycol-bis(allyl carbonate), more currently known as CR39. However, such heating may impair the polarizing film in an uncontrolled manner: the tint of the polarizing film may vary and/or the polarization efficiency of the film may be lowered after the process, and/or the dimensional stability of the film may be impaired. Such a dimensional variation, even if slight, and/or a slight deficiency in the gripping of the film may induce strains in the polarizing film that generate polarization nonuniformity over the entire surface of the film. Furthermore, the position of the polarizing film inside the mold may be affected by the method of filling the mold with the monomer liquid on either side of the film so that, after polymerization, the position of the film relative to the front face of the lens may vary uncontrollably from one region of the optical element obtained to another. Such variations may impair the precision that is needed for certain applications, such as the manufacture of an ophthalmic lens. Moreover, it is particularly difficult with this method to ensure that there is parallelism between one face of the mold and the surface of the polarizing film. This proves to be penalizing for the manufacture of polarizing ophthalmic lenses of the progressive type in which the radius of curvature varies continuously on the surface of the optical element.
The second method of manufacturing an optical element with a polarizing film is used when the optical base element is made of a thermoplastic such as, for example, polycarbonate, polyamide or polymethyl methacrylate (PMMA). The polarizing film is placed on the inside of an injection-molding mold, against one face of said mold, and then the heated thermoplastic is injected under pressure. The temperature of the injected material at the moment it is introduced into the mold is high, typically for example between 270° C. and 320° in the case of a polycarbonate material. In order to obtain a polarizing optical element of acceptable quality in this way when the material of the uniaxially drawn polarizing film is based on PVA, it is necessary for the PVA polarizing film to be initially laminated between two protective films, for example two polycarbonate films each 0.4 mm in thickness. The films protecting the PVA provide the mechanical cohesion that allows the PVA polarizing film to withstand the high injection pressure, and they also have a role of thermal protection against the high temperature at which the thermoplastic is injected. This role is better fulfilled the higher the thickness of the protective film. However, it should be noted that during the injection the polarizing film may undergo uncontrolled deformations, while at the same time it may be impaired by the high temperature of the injected material. In addition, it is important that one of these films also ensures good cohesion with the injected material of the optical element, for example by fusing with the latter on their contact surface. Furthermore, the protective film that covers the polarizing film on the opposite side to the optical element made of injected material must not exhibit substantial birefringence since, in this case, the polarizing efficiency of the optical element that is finally obtained may be reduced. This limits the variety of polarizing film that can be used in this type of method.
One object of the present invention therefore consists in proposing an inexpensive polarizing optical element that can be manufactured simply and without a prolonged heating step, and which exhibits good optical quality by controlling the distance separating the polarizing film from the face closest to the optical element, including for the manufacture of progressive ophthalmic lenses.